Open loop current sensors typically include a magnetic field sensor mounted in the air gap of a magnetic core. A conductor passing through the magnetic core produces a magnetic field comparable to the current. The magnetic field is concentrated by the core and measured by the magnetic field sensor. The current sensor typically amplifies the signal from the magnetic field sensor, and it is this amplified signal that is the output of the sensor. A commonly used magnetic field sensor is a Hall Effect sensor integrated into an ASIC with terminals for its connection to a printed circuit for processing the signals. Open-loop current sensors can also have circuitry that provides temperature compensation and calibrated high-level voltage output.
In three-phase conduction systems, electric current may be measured by positioning sensors around two of the three phases, based on the assumption that the sum of the currents of the three conductors is zero. However, a current sensor can be placed around each of the three conductors, for example to detect possible leakage currents.